The present invention relates to a method of producing high-purity polycrystalline silicon using a silane (SiH4) gas as a raw material, which is used in large quantities, for example, as substrate materials of semiconductor devices.
Since high-purity silicon began to be used for semiconductor devices, longer than 40 years have already passed. During this period, by being supported by the remarkable progress of semiconductor devices, the utilization field of high-purity silicon has expanded, and the demand thereof has greatly increased. Also, with the increase of the performance of semiconductor devices, the high-increase of the quality of a silicon wafer which is a substrate thereof has been required, and further, the more increase of the quality of polycrystalline silicon as a raw material thereof has been required.
High-purity polycrystalline silicon for semiconductor is melted, and after being remade in a state of a single crystal, is used for a semiconductor device. As a method for the production of single crystal silicon, there are a floating zone melting method (hereinafter, is referred to as FZ method) and a Czochralski method (hereinafter, is referred to as CZ method), and forms of polycrystalline silicon used for these methods are different from each other. That is, in the FZ method, polycrystalline silicon produced in a rod form having a diameter of several tens mm or larger and a length of from 1 to 2 m is used, and in the CZ method, nugget silicon formed by breaking the above-described rod-form polycrystalline silicon is used.
Because in the FZ method, a vessel such as a quartz crucible as used in the CZ method is not used, and molten silicon is single-crystallized in a state of being supported by the own surface tension, the quality of the polycrystal largely influences the single crystallization. Also, conventionally, the FZ method is carried out at least twice as is shown in JP-B-45-19562 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined published Japanese patent applicationxe2x80x9d). That is, by at least one preliminary FZ method, defects during the growth of polycrystal are vanished, and in at least one subsequent second FZ method using a seed crystal, a single crystal is obtained. However, because the FZ method is carried out at least twice, because of the FZ apparatus cost, the FZ operation cost (expenses and labor), and the material loss for the polycrystal regulation occurring in carrying out the FZ method, etc., the cost becomes very high. Accordingly, recently, a method of producing a polycrystalline rod capable of achieving single crystallization by using only one time of the FZ method has been strongly desired.
In the defects existing in polycrystalline silicon, the main ones giving bad influences at the single crystallization by the FZ method are an abnormal growth occurred near the interface between a silicon core and polycrystalline silicon newly grown on the surface thereof and the porosity created accompanied by the abnormal growth. About the defects, as shown, for example, in JP-B-56-45852, a method of reducing the defects near the interface between the surface of the silicon core and the newly grown polycrystalline silicon by lowering, particularly, the reaction temperature under the thermal decomposition conditions at the beginning of the growth of polycrystal than the temperature at an ordinary thermal decomposition by several tens xc2x0 C., and other efforts for solving the problems have been made for many years, and improvements have been made a little by little.
However, as the result of investigating the mechanism of the deposition of polycrystalline silicon by the thermal decomposition of a silicon hydrides over several tens years, the inventors have recognized that the occurring ratio of the defects near the above-described interface changes according to the cleanness and the ununiformity of the surface of the silicon core, and in the case of production on an industrial scale, a silicon core having a good quality for surface chemistry, it is very difficult to completely vanish the defects near the above-described interface thereby.
Also, the semiconductor product produced from a single crystal by the FZ method is generally required to have a low oxygen concentration as compared with a semiconductor product produced from a single crystal by the CZ method.
The present invention has been made aiming at the above-described problems in the prior art, and an object of the invention is to provide a method of producing high-purity silicon capable of preventing the occurrence of the defects such as abnormal growth in the vicinity of the interface between the silicon core and polycrystalline silicon to be newly deposited on the surface thereof in the case of producing polycrystalline silicon by the thermal decomposition of a silane gas.
In order to achieve the above-described object, a first aspect of a method of producing rod-form high-purity polycrystalline silicon according to this invention comprises depositing silicon on a silicon core by a thermal decomposition of a silane gas, wherein in applying to a silicon core a hydrogen treatment before depositing silicon, the heating temperature and the heating time of the silicon core are controlled according to a dew point of a hydrogen gas filled in a thermal decomposition reactor.
According to the first aspect of this invention, it is presumed that by controlling the heating temperature and the heating time at the hydrogen treatment for the silicon core according to the dew point of the hydrogen gas, the active sites interspersed on the surface of the silicon core are reacted with a vapor of water in the hydrogen gas to become a passive state, whereby the surface-chemically uniform surface of the silicon core is obtained. And thereby, the occurrence of the defects such as abnormal growth and porosity in the vicinity of the interface between the silicon core and polycrystalline silicon to be newly deposited onto the surface thereof can be restrained.
According to a second aspect of this invention, there is provided a method of producing high-purity polycrystalline silicon of the first aspect, wherein when the dew point of the hydrogen gas filled in the thermal decomposition reactor is high, a heat treatment of a high temperature and a short time is applied to the silicon core in the hydrogen gas.
According to the second aspect of this invention, in order make sure the passivation of the whole silicon core, when the dew point of the hydrogen gas is high, that is, when the content of vapor of water contained in the hydrogen gas is relatively high, by applying a hydrogen treatment of a high temperature and a short time to the silicon core, the passivation of the active sites interspersed on the surface of the silicon core can be accelerated, and also the increase of the oxygen concentration can be restrained.
Also, according to a third aspect of this invention, there is provided a method of producing high-purity polycrystalline silicon of the first aspect, wherein when the dew point of the hydrogen gas filled in the thermal decomposition reactor is low, a heat treatment of a low temperature and a long time is applied to the silicon core in the hydrogen gas.
According to the third aspect of this invention, when the dew point of the hydrogen gas is low, that is, when the content of vapor of water contained in the hydrogen gas is relatively low, by applying a hydrogen treatment of a low temperature and a long time to the silicon core, the passivation of the active sites interspersed on the surface of the silicon core can be accelerated.
According to a forth aspect of this invention, there is provided a method of producing high-purity polycrystalline silicon of the first aspect, wherein after finishing the hydrogen treatment of the silicon core, the hydrogen gas in the thermal decomposition reactor is replaced with a high-purity hydrogen gas, and thereafter, the production of polycrystalline silicon by the thermal decomposition of the silane gas is carried out.
According to the forth aspect of this invention, since after passivating the whole surface of the silicon core, the hydrogen gas used in the hydrogen treatment is replaced with a high-purity hydrogen gas, the thermal decomposition of the silane gas to be successively carried out can be proceeded without hindrance. Also, because by this treatment, the oxygen concentration in polycrystalline silicon formed is reduced, the single crystal by the FZ method using the polycrystalline silicon is suitable for the production of a semiconductor product.